Recently, a lightweight secondary battery of a high energy density is being developed as a power supply for portable electronic equipment, such as a notebook portable computer, an information terminal device (Personal Digital Assistant or PDA), a mobile phone, or a video tape recorder (VTR) with a built-in camera. As the secondary battery having this high energy density, there is known a lithium ion secondary battery, having an energy density higher than that of e.g. a lead or nickel/cadmium battery, and in which the charging/discharging reaction of the battery proceeds by allowing lithium ions to be moved between the cathode and the anode.
This lithium ion secondary battery can be further reduced in thickness and weight by employing a film-shaped exterior material in place of e.g. a metal vessel. Specifically, a hermetically sealed structure can readily be obtained in the lithium ion secondary battery by employing, as the film-shaped exterior material, a laminate film composed of a heat-sealable film of a high molecular material, and a metal foil, layered together, and by heat-sealing the laminate film having enclosed therein e.g. the electrodes (see for example the cited document 1).
[Cited Document 1]
Japanese Patent Application Laid-Open No. H8-83596
However, the laminate film used as the exterior material in the above-described lithium ion secondary battery is low in tenacity as compared to other exterior materials, such as metal vessel. Thus, in case the lithium ion secondary battery is over-charged or over-discharged by mistaken operations on the electronic equipment, or is left in a car during summer at a high temperature, the non-aqueous electrolytic solution in the battery is decomposed due to heat, etc., applied to it, to generate e.g. carbonic gas to deform the film-shaped exterior material. The gas evolved in the battery tends to cause the film-shaped exterior material to be expanded like a balloon.